I.e., the suggestion I'm hearing, if I understand right, is that with too large time gaps between classes, people will not significantly progress at all -- not just much more slowly, but not at all. That even if they train for several hundreds of hours, they will not improve significantly if they leave too many days between classes. I assume the idea is that the brain (or body?) tends to keep forgetting everything learned between classes, such that the person tends to just keep relearning what they know, rather than continuing to progress?

I think that's generally true. I think it depends on the individual's prior training, though. Someone who trains five days a week for a period of years and then drops back to two days a week when life interferes is going to be in a much different place than a beginner who only trains for two days a week from the very beginning. You need some kind of foundation in order to make progress, and I don't think beginners can build that foundation in two classes a week.

At the other extreme, we have shihans who are almost exclusively teaching other people -- most of whom are vastly less skillful -- and who still continue to learn and improve. I think it's safe to say that such people have internalized the art to such a degree that they are their own best teachers. "Mat time" is irrelevant, because their whole life is their practice.

I think it's safe to say that most people in most dojos are closer to the untrained beginner than the shihan end of things.